package cn.gem.rainbow.cfg.web.filter;

import lombok.extern.log4j.Log4j2;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;

/**
 * Filter是J2E中来的，可以看做是Servlet的一种“加强版”，它主要用于对用户请求进行预处理和后处理，拥有一个典型的处理链。Filter也可以对用户请求生成响应，这一点与Servlet相同，但实际上很少会使用Filter向用户请求生成响应。
 * 使用Filter完整的流程是：Filter对用户请求进行预处理，接着将请求交给Servlet进行预处理并生成响应，最后Filter再对服务器响应进行后处理
 * <p>
 * 过滤器的使用场景
 * 1) Authentication Filters, 即用户访问权限过滤
 * 2) Logging and Auditing Filters, 日志过滤，可以记录特殊用户的特殊请求的记录等
 * 3) Image conversion Filters
 * 4) Data compression Filters
 * 5) Encryption Filters
 * 6) Tokenizing Filters
 * 7) Filters that trigger resource access events
 * 8) XSL/T filters
 * 9) Mime-type chain Filter
 */
@Log4j2
public class ReqTimeFilter implements Filter {


    /**
     * 完成Filter的初始化
     * <p>
     * Called by the web container to indicate to a filter that it is being
     * placed into service. The servlet container calls the init method exactly
     * once after instantiating the filter. The init method must complete
     * successfully before the filter is asked to do any filtering work.
     * <p>
     * The web container cannot place the filter into service if the init method
     * either:
     * <ul>
     * <li>Throws a ServletException</li>
     * <li>Does not return within a time period defined by the web
     *     container</li>
     * </ul>
     * The default implementation is a NO-OP.
     *
     * @param filterConfig The configuration information associated with the
     *                     filter instance being initialised
     * @throws ServletException if the initialisation fails
     */
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        log.info("初始化过滤器：{}", filterConfig.getFilterName());
    }

    /**
     * 实现过滤功能，即对每个请求及响应增加的额外的预处理和后处理。
     * ,执行该方法之前，即对用户请求进行预处理；执行该方法之后，即对服务器响应进行后处理。
     * <p>
     * 值得注意的是，chain.doFilter()方法执行之前为预处理阶段，该方法执行结束即代表用户的请求已经得到控制器处理。
     * 因此，如果在doFilter中忘记调用chain.doFilter()方法，则用户的请求将得不到处理
     * <p>
     * The <code>doFilter</code> method of the Filter is called by the container
     * each time a request/response pair is passed through the chain due to a
     * client request for a resource at the end of the chain. The FilterChain
     * passed in to this method allows the Filter to pass on the request and
     * response to the next entity in the chain.
     * <p>
     * A typical implementation of this method would follow the following
     * pattern:- <br>
     * 1. Examine the request<br>
     * 2. Optionally wrap the request object with a custom implementation to
     * filter content or headers for input filtering <br>
     * 3. Optionally wrap the response object with a custom implementation to
     * filter content or headers for output filtering <br>
     * 4. a) <strong>Either</strong> invoke the next entity in the chain using
     * the FilterChain object (<code>chain.doFilter()</code>), <br>
     * 4. b) <strong>or</strong> not pass on the request/response pair to the
     * next entity in the filter chain to block the request processing<br>
     * 5. Directly set headers on the response after invocation of the next
     * entity in the filter chain.
     *
     * @param request  The request to process
     * @param response The response associated with the request
     * @param chain    Provides access to the next filter in the chain for this
     *                 filter to pass the request and response to for further
     *                 processing
     * @throws IOException      if an I/O error occurs during this filter's
     *                          processing of the request
     * @throws ServletException if the processing fails for any other reason
     */
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        log.info("start to doFilter");
        log.info("" + getClass() + " 对请求参数等进行预处理" );
        long startTime = System.currentTimeMillis();

        chain.doFilter(request, response);

        long endTime = System.currentTimeMillis();
        log.info("" + getClass() + " 对响应结果进行预处理" );
        log.info("the request of {} consumes {}ms.", getUrlFrom(request), (endTime - startTime));
        log.info("end to doFilter");


    }


    /**
     * Filter销毁前，完成某些资源的回收
     * <p>
     * Called by the web container to indicate to a filter that it is being
     * taken out of service. This method is only called once all threads within
     * the filter's doFilter method have exited or after a timeout period has
     * passed. After the web container calls this method, it will not call the
     * doFilter method again on this instance of the filter. <br>
     * <br>
     * <p>
     * This method gives the filter an opportunity to clean up any resources
     * that are being held (for example, memory, file handles, threads) and make
     * sure that any persistent state is synchronized with the filter's current
     * state in memory.
     * <p>
     * The default implementation is a NO-OP.
     */
    @Override
    public void destroy() {
        log.info("销毁过滤器");
    }

    private String getUrlFrom(ServletRequest servletRequest) {
        if (servletRequest instanceof HttpServletRequest) {
            return ((HttpServletRequest) servletRequest).getRequestURL().toString();
        }

        return "";
    }
}
